1. Field of the Invention
The invention relates to a plasticizer-containing film suitable as interlayer in laminated safety glass, the film being composed of at least two individual films based on plasticizer-containing polyvinyl acetals having a different polyvinyl acetate content and exhibiting sound-absorbing characteristics.
2. Description of the Related Art
Laminated safety glass generally consists of two glass panels and an intermediate film, which bonds the glass panels together. Plasticizer-containing, partially acetalized polyvinyl alcohol (polyvinyl acetal), in particular polyvinyl butyral (PVB) is predominantly used as a film material. Laminated safety glass (LSG) is used for example as windshields or side windows in the automotive sector and as safety glass in the construction sector.
A feature of laminated glass that becomes more and more important is its sound-damping characteristics. This can be achieved for example by an interlayer film that is particularly soft and hence sound-absorbing. However, the mechanical stability of these films is often insufficient and they do not exhibit adequate adhesion to glass.
Alternatively, multilayer systems can be used in which the individual layers differ with respect to their mechanical strengths, and thus sound absorption is achieved by mechanical decoupling.
Different mechanical strengths of films based on plasticizer-containing polyvinyl acetal can be adjusted for example via their plasticizer content or the portion of polyvinyl alcohol or polyvinyl acetate groups in the polyvinyl acetal used.
For example, U.S. Pat. No. 5,340,654 describes a multilayer system in which one layer comprises a PVB having a residual acetate content of 8 to 30 mol % and a second layer comprises a PVB having a residual acetate content of less than 4 mol %. U.S. Pat. No. 5,340,654 indicates that PVB having a residual acetate content of less than 8 mol % is unsuitable for sound absorption.
WO 2006/102049 discloses a similar multilayer system in which both PVB partial films have a residual acetate content of less than 5 mol %, different portions of polyvinyl alcohol groups and a different plasticizer content and hence different mechanical strengths.
The production of polyvinyl butyral customarily used for interlayer films occurs on an industrial scale by saponification of polyvinyl acetate to polyvinyl alcohol with desired residual acetate content and the subsequent reaction thereof with an aldehyde to the corresponding polyvinyl acetal with the desired degree of acetalization.
Polyvinyl alcohol is produced from polyvinyl acetate by direct hydrolysis or by alcoholysis. The reaction can be catalyzed by strong acids or bases (cf. Encyclopedia of Polymer Science and Engineering, Wiley, New York 1989, Vol. 17). On an industrial scale, the alcoholysis of polyvinyl acetate with methanol with alkaline catalysis to form methyl acetate and polyvinyl alcohol is preferred. Excess solvent and methyl acetate are removed by distillation after the desired degree of hydrolysis is reached. The degree of hydrolysis of the polyvinyl alcohol can be adjusted via retention time, catalyst concentration and temperature. The production cost of polyvinyl alcohol generally increases with increasing degree of hydrolysis, since on the one hand the yield of polyvinyl alcohol based on polyvinyl acetate decreases and on the other hand catalyst quantity, retention time and quantity of solvents to be distilled increase.
The higher the degree of hydrolysis of the polyvinyl alcohol, the higher are also the process costs in the production of a polyvinyl acetal. On the one hand, this is due to the cost of the polyvinyl alcohol itself that is increasing with the degree of hydrolysis (see previous section). On the other hand, the required amount of aldehyde to obtain a predetermined content of OH groups (expressed as polyvinyl alcohol content of the polyvinyl acetal in percent by weight) increases with the degree of hydrolysis.
The systems described in prior art for multilayer films comprise polyvinyl acetals having a low residual acetate content, i.e. the polyvinyl acetals are made from highly saponified polyvinyl alcohols. This is disadvantageous for the energetic and economical reasons mentioned above.